Insulating

[steq184]

We are moving into an old house very soon, the house has a seperate double garage with a pitched roof with 3 single rooms attached to it.
The construction is block wall and felt roof.
We want to make this a bit more habitable so we can use it all year round.
My idea was to block up the garage door, fit batterns to the inside of all the exterior walls at 450mm gaps and fill with insulation and then cover with plasterboard.
I am also going to plasterboard out the ceilings, create a loft hatch so I can fill with insulation.
Am either going to felt, shingle or fit lightweight steel tiles to the roof......depending on price.
I do not know what sort of damp proof the building has, what I am afraid of is after doing all this that later on I get the smell of mould and cannot see it as its behind the new plasterboard so have to rip all out looking for it.
What can I do to stop or remedy this.....is there something I can paint on the existing walls etc?
Other questions are:
What insulation to use for walls. Was looking at Kingspan but I think its out of my budget to be honest. What about Rockwall or Polystyrene?
What type of plasterboard for walls and ceilings. Foil backed etc?
Do I need a vapour barrier anywhere.
What would I need to do to the floor?

I really appreciate your help on this.

[stoneyboy]

steq184,
If you fit insulation against the walls without a ventilated gap you will almost certainly get damp problems.
You need to build a box inside the garage with a ventilating gap on all sides.
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